1. Field of the Invention
The present invention relates to a gain control device for performing gain control of received signals, and particularly to a gain control device suitable for reception of packet signals.
2. Related Art
It is proposed to employ communication using packets (i.e., communication in which signals are transmitted/received at random and non-predetermined times, instead of communication using time slots (i.e., communication in which signals are transmitted/received at predetermined times), as a means for matching in a computer network.
Further, communication techniques such as QAM or OFDM, in which the amplitude of a signal will greatly vary, are proposed for improving a transmission rate.
In a digital receiver, signals are sampled by an A/D converter. In order to obtain signals accurately, it is desirable to make full use of the dynamic range of the A/D converter. On the other hand, particularly in mobile communication systems, received power will widely vary depending on the distance between communication terminals. Therefore, in order to make full use of the dynamic range of the A/D converter as described above, a gain control device capable of controlling received power to provide constant power regardless of the magnitude of the received power is needed at the input side of the A/D converter.
In order for the gain control device to be applied to packet communication, it should detect a packet signal transmitted at random and non-predetermined times, and perform gain control at high speed for immediately regulating the signal to the constant output power. However, the signal wave will be distorted if the gain is controlled in direct response to variation in the signal amplitude. Thus, in this case, it should perform gain control of the signal at low speed.
JP-A-9-307601 proposed a gain control device for gain control of QAM transmitting waves. In this device, a component of envelope detection of transmitting waves (i.e., voltage values corresponding to power) is sampled-and-held and integrated for performing gain control. Further, in this device, the period of sampling-and-holding is set short immediately after the start of transmitting so that a control rate is made higher. After a while, the period is set longer in order to make the control rate lower. This technique is limited to transmitting waves. In the case of transmitting waves, control can be accomplished easily since the time when a signal is transmitted is known. However the technique is not applicable to received signals without modification. Particularly it is not applicable to received signals in packet communication because the time when a signal is received is unknown. Further, this device sample-and-holds a signal and generates step waves before passing the waves through an integration circuit for generating a control signal. In this case, compared to the case of integrating continuous waves, quantization errors are produced and as a result control precision is lowered.